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To the naked eye, the white puffs of cotton growing on shrubs, the yellow flowers on canola plants and the towering tassels on cornstalks look just like those on any other plants. But inside their cells, where their DNA contains instructions for how these crops should grow, there are a few genes that were put there not by Mother Nature but by scientists in a lab.

Some of the genes are from a soil bacterium called Bacillus thuringiensis that makes proteins lethal to flies, moths and other insects. Others are from the soil bacterium Agrobacterium that programs plants to make a key enzyme that isn't vulnerable to a popular weed killer. These modifications allow farmers to grow crops with easier weed control and fewer pest-killing chemicals.

To an increasingly vocal group of consumers, this genetic tinkering is a major source of anxiety. They worry that eating engineered foods could be bad for their health or cause unanticipated environmental problems. At the very least, they insist, they deserve the right to know whether the foods they might buy contain genetically modified ingredients.

In California, this unease has culminated in Proposition 37. If approved on Nov. 6, the initiative would require many grocery store items containing genetically modified ingredients to carry labels.

But among scientists, there is widespread agreement that such crops aren't dangerous. The plants, they say, are as safe as those generated for centuries by conventional breeding and, in the 20th century, by irradiating plant material, exposing it to chemical mutagens or fusing cells together to produce plants with higher grain yields, resistance to frost and other desirable properties. Now they want to insert other genes into plants to make them more nutritious, resistant to drought or able to capture nitrogen from the air so they require less fertilizer, among other useful traits.

"There's no mystery here," said UCLA plant geneticist Bob Goldberg. "When you put a gene into a plant ... it behaves exactly like any other gene."

Genetically engineered crops have been extensively studied. Hundreds of papers in academic journals have scrutinized data on the health and environmental impacts of the plants. So have several in-depth analyses by independent panels convened by the National Academy of Sciences.

The reports have broadly concluded that genetically modified plants are not only safe but in many respects friendlier to the environment than nonengineered crops grown via conventional farming methods.

For instance, a review this year of 24 long-term or multigenerational studies found that genetically modified corn, soy, potato, rice and wheat had no ill effects on the rats, cows, mice, quails, chickens, pigs and sheep that ate them. Growth, development, blood, tissue structure, urine chemistry and organ and body weights were normal, according to the report in Food and Chemical Toxicology.

About 90% of the corn, soy and cotton now grown in the U.S. is genetically modified, and that has led to less use of pesticides, more targeted insect control, a shift to fewer toxic chemicals and less soil erosion compared with conventional farms, according to a 250-page analysis from the National Academies in 2010.

"There were hundreds and hundreds of peer-reviewed articles we combed through," said environmental economist David Ervin of Portland State University, who chaired the panel.

Though genetically modified crops are widespread, the alterations are quite limited.

The most common one makes crop plants tolerate the herbicide Roundup, allowing them to thrive while weeds die. Roundup kills weeds by disabling an enzyme called EPSPS that plants need to make amino acids. But crops are vulnerable too. So scientists at Monsanto Co. developed seeds with a resistant version of the EPSPS gene from Agrobacterium, splicing it into soy, alfalfa, corn, cotton, canola and sugar beets. The resulting crops have built-in protection to the herbicide; hence the brand name Roundup Ready.

It was such an easy way to control weeds that farmers flocked to it, said weed scientist Mike Owen of Iowa State University in Ames: "The siren song of simplicity and convenience was incredibly powerful."

Scientists used another strategy to make crops that can resist insect pests, such as the European corn borer and cotton bollworm.

For this job, the key genes are from Bacillus thuringiensis, known as Bt, which makes proteins that are toxic to insects but harmless to fish, birds, people and other vertebrates because they lack a receptor to which the proteins bind.

For decades, Bt proteins have been sprayed on organic crops to control insects. In the genetically modified version of the strategy, genes for Bt proteins are spliced into the plant's DNA so that it makes the protein itself.

Adoption of these crops has led to several documented benefits. American farmers cut back on their use of traditional insecticides that kill a broader array of bugs — including helpful ones — between 1996 and 2008, the National Academies review found.

China's broad adoption of Bt cotton led to a rise in numbers of beneficial ladybugs, lacewings and spiders and fewer aphids and other pests, according to an April study in the journal Nature. Benefits like these spill over to conventional crops as well, scientists have found.

In one famous case, genetic engineering saved a crop headed for extinction. Papaya plantations in Hawaii were under attack from the papaya ringspot virus; a new genetically altered papaya is resistant.

None of this means that modified crops are perfect. Problem weeds like waterhemp and Palmer pigweed are developing resistance to Roundup around the U.S., undercutting the usefulness of Roundup Ready crops.

That doesn't surprise Owen, who saw the same thing happen with older herbicides for conventional crops. The reason was the same: overuse of one chemical. The solution, he said, is not to ditch engineered crops but to incorporate them with a variety of herbicides, cover crops, crop rotation and tilling of the soil.

He said he gets irked by talk of monster "superweeds" spawned by genetically modified crops: "It was the decision of how the genetic engineering was going to be used that created the problem."

To discourage the evolution of pests that are resistant to Bt proteins, the Environmental Protection Agency requires farmers to plant a buffer zone of conventional crops near ones engineered for resistance. Farmers have been lax about this, undermining the technology's usefulness, said Gregory Jaffe, director of the biotechnology project for the Center for Science in the Public Interest in Washington.

"If it's an environmentally beneficial pesticide, we want to protect it," he said.

Indeed, reports of Bt-resistant pests are cropping up, just as occurred with traditional insecticides, said entomologist Yves Carriere of the University of Arizona in Tucson. To stave off the trend, companies are creating crops with multiple Bt genes, since it's harder for insects to develop resistance to all of them at once.

Engineered crops can — and do — cross with conventional crops, creating occasional embarrassments for the plant biotech industry and headaches for organic farmers who want their products free of genetically modified ingredients. In Europe, rules state that products can be labeled alteration-free if they contain up to 0.9% genetically modified content. There is no such cutoff in the U.S., Carriere said.

Stacy Malkan, a spokeswoman for California's Yes on 37 campaign, said she and others were not convinced by the safety data on genetically modified crops because, among other reasons, many of the studies were done by industry scientists and didn't assess effects of eating such crops for a long enough period of time.

It's also a matter of basic consumer rights, she added.

"When we're the ones buying the food, we should get to know what we want to know about it," she said.

As the wife of an organic farmer, Pamela Ronald is heartened that consumers are interested in food safety and sustainability. But as a UC Davis plant geneticist, she said the labels required by Proposition 37 wouldn't tell people what they want to know.

"It has no meaning, whether it's [genetically modified] or not," she said.